It is known in the art of processing sewage sludge to render the sludge safe and sanitary by various techniques, a number of which have been approved by the Environmental Protection Agency EPA, which agency has developed regulations for proper treatment and disposal of sewage sludge.
The goal of treating sewage sludge is to neutralize pathogens to an environmentally safe level and to reduce vector attractiveness; i.e., to make the sewage sludge unattractive to rats, mice, flies, birds, etc., which could serve as vectors for bringing the pathogens in sewage sludge into contact with humans and thus pose a risk to human health.
Various apparatus and methods for killing pathogens and reducing vector attractiveness have been developed and some examples of such apparatus and methods are set forth in U.S. Pat. Nos. 5,013,458, 5,229,011, 5,186,840, 5,405,536, 5,433,844, 5,554,279, and 5,681,481, the complete disclosures of all of which are incorporated herein by reference.
Some previous developments in the treatment of sewage sludge have sought to inexpensively stabilize the sludge through lime addition. These systems sometimes produced objectionable odors, dust and steam while producing an end product that was of a pasty consistency and therefore difficult to handle often requiring specialized spreading equipment, for spreading the resultant treated waste on land. Additionally, with some prior art systems, objectionable odors have prevented the wider adoption of these systems.
Other processes used for treating sewage sludge include composting. These processes require the addition of bulking agents to the sewage sludge in preparation for the composting process. Frequently, the bulking agents need to be removed using specialized equipment before the resulting treated biosolids can be land applied, thus increasing the cost and complexity of the treatment process.
In accordance with the existing developed technology, drying apparatus of various forms have been used to stabilize sewage sludge and produce a granular end product that appeared to be satisfactory, but was so extremely dry, for example in excess of 90% dry solids, such that the end product was often dusty and difficult to handle. Such processes and equipment lacked the ability to determine the solids concentration with the required degree of precision, in that they simply evaporated water until the product became very dry.
Additionally, some current apparatus and processes that are in use often estimate the moisture content of the final product in an indirect manner, using indirect measurements or timers. Consequently, the material being processed is dried until the temperature of the medium providing the heat increases rapidly, providing an indication that all of the moisture has been removed from the product. Thus, in such processes and equipment, the processing of the batch is then considered to be complete, although it can be extremely dry and difficult to handle.